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Potato is considered a nitrogen (N) intensive plant with a low N use efficiency (NUE). The current study introduced an excellent approach by combining dicyandiamide (DCD), moringa seed oil (MSO), or zeolite (ZE), with N fertilizer for maximizing potato tuber yields and NUE as well as minimizing tubers nitrate (NO 3 - ) accumulation. The impact of these materials on soil N availability and gaseous emissions (NH 3 , and N 2 O) was investigated under incubation conditions. A 2-year field experiment were carried out with seven treatments [without N (control), N fertilizer (350 kg N-urea ha -1 as a recommended dose; Urea RD ), 75% of N recommended dose with DCD (Urea 75%RD +DCD), Urea 75%RD with 2% MSO (Urea 75%RD +MSO 2% ), Urea 75%RD with 4% MSO (Urea 75%RD +MSO 4% ), Urea 75%RD with 0.5 Mg ZE ha -1 (Urea 75%RD +ZE R1 ), and Urea 75%RD with 1.0 Mg ZE ha -1 (Urea 75%RD +ZE R2 )]. We also conducted a 40-days incubation trial with the same treatments; however, urea was added at the rate of 200 mg N kg -1 soil for all treatments, excluding the control. The addition of DCD, MSO, and ZE with urea under incubation conditions delayed the nitrification process, thereby causing a rise in NH 4 + -N content and a decrease in NO 3 - -N content. Ammonia-oxidizing bacteria (AOB) was inhibited (p ≤ 0.01) in treatments Urea+DCD, Urea+MSO 4% , and Urea+ZE R2 . The highest NUE indexes were recorded in treatment Urea 75%RD +DCD. The highest NO 3 - accumulation (567 mg NO 3 - kg -1 ) in potato tubers was recorded in treatment Urea RD . Whilest, the lowest NO 3 - content (81 mg NO 3 - kg -1 ) was in treatment Urea 75%RD +DCD. The lowest cumulative N 2 O emissions and highest cumulative NH 3 volatilization were observed in the treatment Urea+DCD under incubation conditions. Our findings demonstrated that N fertilizer rate could be reduced by 25%, while the tuber yields increased with an acceptable limit of NO 3 - content, resulting in economical, agronomical, and environmental benefits., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Nitrification inhibition as an alleviation tool to decrease nitrogen (N) losses and increase N use efficiency (NUE) as well as reducing NO 3 accumulation in plants is a promising technology. No study thus far has directly or indirectly to use the secondary metabolites extracted from Moringa (Moringa oleifera Lam) seeds as nitrification inhibitors. Moringa seed extract (MSE) was studied based on its content of phenolic compounds (PC) and for its antioxidant characteristic. A 2-year field experiment and 30-day incubation experiment were conducted with three treatments of control (CK), N fertilizer (300 kg N ha - accumulation in plants is a promising technology. No study thus far has directly or indirectly to use the secondary metabolites extracted from Moringa (Moringa oleifera Lam) seeds as nitrification inhibitors. Moringa seed extract (MSE) was studied based on its content of phenolic compounds (PC) and for its antioxidant characteristic. A 2-year field experiment and 30-day incubation experiment were conducted with three treatments of control (CK), N fertilizer (300 kg N ha -1 and 200 mg N kg -1 soil for the field and incubation experiment, respectively), and N fertilizer with MSE (500 ppm as a TPC) to investigate the responses of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to MSE and the consequences for NUE and NO 3 - accumulation in potato tubers. Total phenolics amount was 144 mg gallic acid equivalent g -1 MSE, while flavonoid contents were 76.6 quercetin equivalent g -1 MSE. MSE showed antioxidant activity that was comparable to the standard antioxidants TBHQ and gallic acid. MSE application with N fertilizer retarded the nitrification process, as indicated by a higher NH 4 + -N and lower NO 3 - -N content, compared with N fertilizer application alone. NH 4 + -N content reduced to initial CK level on Day 20 under N fertilizer application alone. However, NH 4 + -N content decreased to initial control level on Day 30 when MSE was applied. The mechanisms resulted from curbing AOB growth by phenolic compounds (TPC, TF, TAC), leading to a delay in nitrification process. AOB increased significantly when N fertilizer was applied alone; on the contrary, AOA was not sensitive to N fertilizer (with and without MSE). Increase in NUE from 37.5% to 66.3% in potato plants under MSE application with N fertilizer was also observed compared with N fertilizer application alone. The highest NO 3 - ). The highest average of N uptake, fresh and dry weight, carotenoids, chlorophyll a, chlorophyll b and nitrate reductase activity was recorded when MSE was applied with N fertilizer. Accordingly, using of Moringa extracted secondary metabolites to suppress AOB growth in the soil is a significant strategy to reduce nitrification rate and N loss from soils, and therefore increase NUE as well as reducing NO 3 - kg -1 ) in tubers was recorded under N fertilizer application without MSE. MSE application significantly decreased NO 3 - accumulation (92 mg NO 3 - kg -1 ) in tubers which is lower than the maximum value of accepting tubers (200 mg NO 3 - kg -1 ). The highest average of N uptake, fresh and dry weight, carotenoids, chlorophyll a, chlorophyll b and nitrate reductase activity was recorded when MSE was applied with N fertilizer. Accordingly, using of Moringa extracted secondary metabolites to suppress AOB growth in the soil is a significant strategy to reduce nitrification rate and N loss from soils, and therefore increase NUE as well as reducing NO 3 - accumulation in potato tubers., (Copyright © 2019 Elsevier Inc. All rights reserved.)